Talking about changes to above mentioned wiki article (web server) it usually takes around 1.5 .. 2 seconds to send (POST) data to server and to receive the updated page, so experimenting delays of 4 .. 10 .. 20 seconds has been quite unusual.

In T297544#7571621, @ema wrote:

@Ade56facc: To further diagnose the problem and figure out whether or not we are looking at two distinct issues it would be useful to know which articles you were editing when getting the errors, or at least their approximate size in bytes. My suspicion is that the problem here is about "large" POST requests timing out.

Title of ticket is wrong, it should be: Image responses without "Last-Modified" and "ETag" HTTP headers.

Around 13..14 (1 .. 2 p.m. UTC) I noticed long delays (6 .. 10 seconds) when trying to "Publish" or "Show preview" (of) an article.

OK, I have seen again responses from server Thumbor without headers named in bug title.

In previous comment last sentence should be:
"... by subtracting initial session time from current time ..."

2. Cookie value sent in HTTP requests changes every minute and this prevents caching of wiki articles for more than 1 .. 60 seconds.

1. Server "Thumbor*", used for static images, sending neither "Last-Modified" nor "ETag" HTTP headers.